Method for generating spread spectrum driver signals for a seismic vibrator array using multiple biphase modulation operations in each driver signal chip

Abstract

A method for generating seismic energy for subsurface surveying include operating a first seismic vibrator above an area of the subsurface to be surveyed and operating at least a second seismic vibrator above the area substantially contemporaneously with the operating the first seismic vibrator. The first and the second vibrators each have a different selected frequency response. The first and second vibrators each is operated by a same direct sequence spread spectrum signal, wherein a different number of modulation operations for each logical value in the direct sequence spread spectrum signal is selected for each vibrator.

1. A method for generating seismic energy for subsurface surveying, comprising:
 operating a first seismic vibrator above an area of the subsurface to be surveyed; and 
 operating at least a second seismic vibrator above the area substantially contemporaneously with the operating the first seismic vibrator, the first seismic vibrator and the at least a second vibrator each having a different selected frequency response range, the first and second vibrators each operated by a same direct sequence spread spectrum driver signal, wherein a different number of modulation operations for each logical value in the direct sequence spread spectrum signal is selected for each vibrator. 
 
     
     
       2. The method of  claim 1  wherein each number of modulation operations is selected to correspond to the frequency response range of each vibrator. 
     
     
       3. The method of  claim 1  wherein the direct sequence spread spectrum signal comprises at least one of a maximum length sequence code, a Gold sequence code and a Kasami sequence code. 
     
     
       4. The method of  claim 1  wherein each vibrator has at least two resonance frequencies in a selected frequency band. 
     
     
       5. The method of  claim 1  wherein the area of the subsurface comprises a body of water, and wherein an actuation time of each vibrator corresponds to a depth at which each vibrator is operated in the body of water such that the first and at least a second vibrators act as a vibrator array. 
     
     
       6. The method of  claim 5  wherein the vibrators each operated at a water depth such that a surface ghost amplifies a downward output of each vibrator within a selected frequency range. 
     
     
       7. The method of  claim 1  wherein the modulation operation comprises biphase modulation. 
     
     
       8. A method for subsurface seismic surveying, comprising:
 operating a plurality of seismic vibrators each having a different selected frequency response range above an area of the subsurface to be surveyed; and 
 detecting seismic signals originating from the plurality of vibrators at each of a plurality of seismic receivers disposed at spaced apart locations above the area; 
 wherein each vibrator is operated by a same direct sequence spread spectrum driver signal; 
 wherein a different number of modulation operations for each logical value in the driver signal is selected for each vibrator; and 
 determining seismic energy originating from each of the plurality of vibrators in the detected seismic signals. 
 
     
     
       9. The method of  claim 8  wherein each number of modulation operations is selected to correspond to the frequency response range of each vibrator. 
     
     
       10. The method of  claim 8  wherein the direct sequence spread spectrum signal comprises at least one of a maximum length sequence code, a Gold sequence code and a Kasami sequence code. 
     
     
       11. The method of  claim 8  wherein each vibrator has at least two resonance frequencies in a selected frequency band. 
     
     
       12. The method of  claim 8  wherein the area of the subsurface comprises a body of water, and wherein an actuation time of each vibrator corresponds to a depth at which each vibrator is operated in the body of water such that the first and at least a second vibrators act as a vibrator array. 
     
     
       13. The method of  claim 12  wherein the vibrators each operated at a water depth such that a surface ghost amplifies a downward output of each vibrator within a selected frequency range.